Scours are sometimes visible in sedimentary rocks, and if you could identify one it allows you to conclude a few things from looking at an outcrop. The most important thing that you can figure out with a scour is which direction the water was flowing as it was created.
These marks in sedimentary rocks are created when the flow of water is suddenly increasing. Mudstone (or sandstone just relatively softer) is deposited when there is little flow of water, and the the clay particles have time to settle to the bottom. As water speed picks up, the flow can knock pebbles onto this muddy surface.
Directly behind the pebbles that lie on the mud surface, a small suction is created where the water picks up higher speeds as it travels around the pebble. This faster moving water in turn begins to erode away the mud behind the pebble. More pebbles tend to fall in these eroding holes, speeding up the process and resulting in a common indicator of scours, the coarse infill. Knowing that the mudstone is below the coarse infill, you can determine stratigraphic up, or which way was vertical when the rocks were deposited and before they were uplifted.
Scours make a very typical shape that allows you to determine the flow direction of the water. On the upstream edge of the scour, the contact between mudstone and infill will be very steep, almost vertical. On the downstream edge, the slope is gentle into the trough. Using this information, you can determine which way the water was flowing, which can provide interesting insight about the history of the rock.
